If you topped up to exactly 5 gallons and used all the extract in the kit, it's impossible for it to be off (sugars just don't disappear- they're in there!) by more than a very little bit. Sometimes the manufacturer of the kit makes the og estimate but uses a different brand of extract which may vary ever so slightly. Still, the biggest cause of incorrect sg readings is that when you topped off with cool water, even if you stir like crazy, the heavier warmer wort sinks to the bottom and you don't get a completely accurate reading because it's more diluted at the top. It doesn't affect the fermentation at all, since the yeast easily can find that sugar to eat! If you used a wine theif and pulled the sample from the bottom, I bet it would have been a little high.
Either way, it's no worry. I'd go with the estimate, unless you used too much water or didn't use all the extract.